State and federal regulations require rigorous inspection of nuclear power plants. Inspections must be carried out both before the plant is put in initial service and subsequently during periodic plant shutdowns. Among the more critical components of a nuclear power plant are the complex stainless steel piping systems which may comprise individual sections of cast stainless steel (CSS) pipe welded together. Such piping systems typically carry reactor coolants which may be radioactive. The integrity of such piping is thus a critical aspect of the safe operation of a nuclear power plant.
Because non-destructive examination (NDE) involving radiographic methods is not practical in the generally radio-active atmosphere within a nuclear power plant, regulations commonly require that ultrasonic testing be employed for initial and subsequent testing of CSS piping components. Ultrasonic testing generally involves launching an elastic wave pulse into the volume of the component being tested and detecting reflected pulses. Typically, soundwaves will be reflected from various geometric reflectors, some of interest, such as flaws to be detected, and some which constitute noise, such as normal geometric boundaries in the structure and acoustical discontinuities in the granular microstructure of the material. In addition, the microstructure of the material, depending upon its nature, may cause distortion or attenuation of the reflected signals further complicating the testing.
The typical flaws or degradation in cast stainless steel piping are the result of intergranular stress-corrosion cracking or fatigue cracks most often occuring in the vicinity of weldments. These flaws may be very tight cracks through the material. The ultrasonic signals reflected from such flaws are often small in amplitude and difficult to distinguish from other reflected signals, particularly the noise resulting from the normal microstructure of the material.
If the character of the microstructure of the material to be tested is known or if calibration specimens having the same structure as the material to be tested are available, the ultrasonic testing apparatus and technique may be adapted by one skilled in the art to optimize flaw detecting efficiency. The problem of applying such techniques to the in situ inspection of CSS piping in nuclear power plants is that the microstructure of the CSS in not uniform. The microstructure of CSS varies from elastically isotropic with equiaxed, relatively small grains to elastically anisotropic with columnar grains oriented radially with respect to the cross-section of the pipe. Variations may occur from one pipe component to another or longitudinally or circumferencially along a single pipe component. In addition, the microstructure may be some combination of the two extremes described.
In order to optimize the ultrasonic flaw detection in CSS piping, it is necessary to first determine the nature of the microstructure at each individual test site. This has not heretofore been possible in an automated ultrasonic scanning device. For example, the microstructure of the pipe may be determined by measuring the skew in the path of a soundwave reflected from the inner wall of the pipe. Since it is known that the velocity of elastic waves depend upon the direction of propagation relative to the axis of the microstructure grains within a material, an anisotropic material will exhibit greater skewing of elastic waves than an isotropic material. This technique, however, requires accurate knowledge of the wall thickness of the pipe. For cast stainless steel pipe used in nuclear power plants, this technique is not useful because the tolerances in the pipe wall thickness are too large.
Because of these difficulties, a conservative approach toward replacement of potentially flawed pipes in nuclear power plants has thus been necessary. Replacement of piping in nuclear power plants is an expensive proposition, particularly when it involves unplanned or extended shut-down of the reactor and consequential loss of revenue for the utility.
Accordingly, it is an object of the present invention to provide a more reliable NDE method for inspection of components of nuclear power plants. It is a further object of this invention to provide an improved method for ultrasonic testing of cast stainless steel piping components in a nuclear power plant. It is still another object of this invention to provide an apparatus for improved in situ ultrasonic testing of cast stainless steel pipe in nuclear power plants. It is another object to provide a method for determining the microstructure of cast stainless steel piping components independent of the dimensions of such components whereby such determination facilitates the choice of a more reliable technique for flaw detection.